Piston pump



FIG. 1 is a front, left perspective view of a piston pump showing our new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a right side view thereof, the left side view being a mirror image thereof;

FIG. 5 is a top plan view thereof; and,

FIG. 6 is a bottom plan view thereof. 

CLAIM The ornamental design for a piston pump, as shown and described. 